Direct Determination of the NaF/AlF₃ Molar Ratio by Raman Spectroscopy in NaF–AlF₃–CaF₂ Melts at 1000 °C

Abstract

For the last 40 years, Raman spectroscopy has been very useful in investigating the structure of corrosive molten salts, such as the cryolite-based melts widely used as electrolyte in the Hall-Heroult process. Even if this process remains the most economically efficient for metallic aluminum electro-production, it suffers from a high energy loss, which is dependent on the melt composition. Therefore, controlling the chemical composition of the electrolyte is essential. The present paper proposes to apply Raman spectroscopy for the direct determination of the NaF-AlF3 molar ratio in NaF-AlF3-CaF2-based melts. Despite the experimental difficulties, a calibration curve based on equilibria taking place in the melt has been developed and the procedure has been successfully compared to industrial samples of known compositions. The possible exportation of the laboratory scale procedure to an industrial environment application for the control of the Hall-Heroult process is finally discussed.

Existence of Al2F7− in molten MF–AlF3 (M = K, Cs) systems as determined by Raman spectroscopy and structural simulation

“…Unfortunately, the bands belonging to these ion clusters are absent from Fig. 1 , which may be because the concentrations and/or the scattering coefficients [2,9] of these ion clusters were too low. Thus, the corresponding Raman bands could not be detected.…”

“…The molten mixtures of alkali metal fluorides and aluminum fluoride are widely applied in the industry. NaF-AlF 3 -based systems are used as electrolytes in industrial aluminum electrolysis [1,2] , while KF-AlF 3 and CsF-AlF 3 molten systems are used as brazing fluxes [3,4] . Furthermore, KF-AlF 3 -based systems are considered as potential aluminum electrolytes [5,6] , particularly when they used in connection with the inert anode [7] and wettable cathode [8] .…”

A review of primary aluminium tapping models

“…In industrial cell, the electrolyte composition is typically close to 80% cryolite (Na 3 AlF 6 ), 10-12% aluminum fluoride (1) 2Al 2 O 3 + 3C = 4Al + 3CO 2 ( AlF 3 ), 2-4% alumina ( Al 2 O 3 ) and 4-7% calcium fluoride CaF 3 on a weight basis [9,10].…”

Determination of the Al2O3 Content in NaF–AlF3–CaF2–Al2O3 Melts at 950 °C by Raman Spectroscopy

“…This procedure allows calculating the cryolitic ratio (CR) with a relative error of 3.5%. 13 It was also discussed that this procedure could possibly be exported to the direct measurement of the CR on a real industrial bath. The procedure relies on the variation of the intensity ratio of the major Raman bands at 560 and 622 cm −1 .…”

“…The sample is held in a small glassy carbon crucible and the excitation laser light hits the melt from the top. More details are available in ref 13. In the present study, for the recording of the Raman spectra, the melts were kept under inert argon atmosphere at 946 ± 5 °C.…”